The active circuit is usually used to improve quality or intensity of signals outputted from the circuit of prior stage before the signals are passed to the circuit of next stage. However, in some applications, for example, when the active circuit is quite close to the signal source, the quality or intensity of the signals received by the active circuit may be already good enough for the circuit of next stage. In this case, to avoid unnecessary power consumption, the active circuit may enable the bypass mode, and may output the received signals directly through the bypassing path of the active circuit. Consequently, the active element of the active circuit can be disabled, and the power consumption can be reduced.
However, since the impedance of the active element was designed to be matched with the signal path during the active mode, the loading effect of the active element may cause the impedance to be no longer matched to the single path. This deteriorates the quality or the intensity of the output signals when the bypass mode is enabled and the active element is disabled. Therefore, it is necessary to design an active circuit that is able to prevent the impedance from being mismatched with the signal path in a bypass mode so as to avoid deteriorating the quality or the intensity of the output signals.